A video signal typically includes vertical display intervals, or fields, having a plurality of horizontal line intervals, e.g. 262.5 lines per field in NTSC video systems. The beginning of each vertical and horizontal interval is identified by respective vertical and horizontal sync pulses that are included in a composite video signal. During a portion of each vertical interval, information in the video signal may not be intended for display. For example, a vertical blanking interval spans approximately the first 20 horizontal line intervals in each field. In addition, several line intervals adjacent to the vertical blanking period, e.g. line 21, may be within an overscan region of a video display and will not be visible.
The lack of displayed image information during blanking and overscan intervals makes it possible to insert an auxiliary information component, e.g. teletext or closed caption data, into these intervals. Standards such as Federal Communications Commissions (FCC) Regulations define the format for each type of auxiliary information including the positioning of the information within a vertical interval. For example, the present closed captioning standard (see e.g. 47 CFR .sctn..sctn. 15.119 and 73.682) specifies that digital data corresponding to ASCII characters for closed captioning must be in line 21 of field 1.
The first step in extracting auxiliary video information is to locate the auxiliary information. Various approaches may be used depending on the type of information involved. For example, recognition of teletext data characteristics such as the framing code pattern is a method of locating teletext data. Closed caption information in line 21 may be located by counting video lines, e.g. counting horizontal sync pulses. Examples of line counting approaches to detecting auxiliary video data may be found in pending International Patent Applications PCT/US/92/04825 and PCT/US/92/04826, filed on Jun. 15, 1992, published on Jan. 21, 1993 (International Publication Numbers WO 93/01680 and WO 93/01681, respectively), and assigned to the assignee of the present application.
After the auxiliary video information is located, the information must be extracted. In the case of digital data, a "data slicer" may be used to convert the video signal into binary data. A data slicer typically operates by comparing the video signal level to a reference level known as the slicing level. For video levels that exceed the slicing level, the comparison produces a logic 1. Video levels that are less than the slicing level produce a logic 0. As an example, closed caption data in line 21 of the video signal may exhibit a signal amplitude range of 0 IRE to 50 IRE. For a signal range of 0 IRE to 50 IRE, a slicing level of 25 IRE would be appropriate.
A constant slicing level may not be adequate for all video signals. Video signal levels may vary depending on the source of the video signal. Utilizing a constant slicing level with varying video signal levels may bias the extracted data undesirably toward logic 0 or logic 1 resulting in erroneous data extraction. For example, if the video signal range is 0 IRE to 20 IRE rather than 0 IRE to 50 IRE, a slicing level of 10 IRE rather than 25 IRE is desirable. If 25 IRE were used as a slicing level for a signal range of 0 IRE to 20 IRE, a logic 1 would never be extracted because the signal never exceeds the slicing level. Thus, it is desirable to adapt the slicing level to the amplitude of the input video signal.
The format of an auxiliary information component such as closed caption data includes provisions to facilitate an adaptive slicing level function. As specified in the FCC standards, a closed caption signal in line 21 begins after the "back porch" interval of the video signal with a 7 cycle burst of a sinusoidal reference waveform designated the "run-in clock" (RIC). The RIC reference component of the auxiliary video data signal is followed in the latter half of the line 21 interval by a data signal component that represents the actual closed caption data. The closed caption data standard establishes that the amplitude of the RIC signal is identical to the amplitude of the data signal. Thus, the average of the RIC signal amplitude is an appropriate slicing level for the subsequent data signal.
An approach to establishing a slicing level based on the RIC signal amplitude is disclosed concurrently-filed U.S. patent application Ser. No. 07/850,481 by E. Rodriguez-Cavazos et al. As disclosed by Rodriguez-Cavazos et al., a slicing level is adjusted to correspond to the average value of the RIC signal amplitude during an interval spanning an integral number of cycles of the RIC signal. The desired interval is defined by creating an averaging window that spans an integral number of cycles of the RIC signal. For example, FCC specifications for closed caption data (see e.g. 47 CFR .sctn..sctn. 15.119 and 73.682) dictate that 7 cycles of a 503 kHz RIC waveform will occur within the RIC signal interval. The duration of one cycle and the RIC interval are approximately 2 .mu.s and 14 .mu.s, respectively. Therefore, as suggested by Rodriguez-Cavazos et al., a 10 .mu.s wide window centered within the RIC interval spans an integral number of cycles, namely 5, as desired. The average value of the amplitude of the RIC waveform during the window is the desired slicing level.
Establishing an accurate slicing level based on the RIC signal requires accurately locating the RIC signal within a line interval that contains auxiliary video data. FCC specifications for closed caption data specify that the RIC signal will begin at approximately 10 .mu.s and end at approximately 24 .mu.s after the leading (falling) edge of the horizontal sync pulse for line 21 of field 1. The FCC specification would appear to permit using a fixed delay from the horizontal sync pulse to accurately locate the RIC signal as required. For example, in the system disclosed by Rodriguez-Cavazos et al., a 10 .mu.s window beginning following a 12 .mu.s delay from the leading edge of horizontal sync spans the time interval from 12 .mu.s to 22 .mu.s after the leading edge of horizontal sync. This window placement is centered within the RIC signal occurring between 10 .mu.s and 24 .mu.s after the leading edge of horizontal sync and would, therefore, encompass an integral number of cycles of the RIG signal.
The described delay approach to locating the RIG signal depends on signal timing that complies precisely with FCC specified values for a composite video signal. In television systems, various versions of horizontal sync signals may be generated. For example, a sync separator may provide sync signals from the composite video signal while a horizontal phase-locked loop (PLL) may produce a uniform sync waveform for deflection purposes. Generating the sync separator output from composite video insures that the sync separator signal is synchronized with the timing of the actual video information in the composite video signal. Under typical conditions, the timing of the horizontal PLL waveform is also synchronized with composite video. Under typical conditions, therefore, either source of sync signals might provide an accurate timing reference for locating a RIC signal within the video signal.
Certain video sources may, however, cause brief but significant timing differences to exist between the composite video sync signal and the output of the horizontal PLL. For example, switching between multiple video read heads in a video cassette recorder (VCR) may produce an abnormal horizontal line period that differs significantly from the nominal 64 .mu.s period. The deviation in the line period may produce a perturbation in the horizontal PLL that is manifested as a substantial phase shift between the composite sync signal at the sync separator output and the horizontal pulse waveform at the horizontal PLL output. The locking action of the PLL gradually corrects the perturbation such that the phase error is substantially eliminated before visible display begins. A significant phase shift may exist, however, for line periods within vertical blanking and overscan intervals. At line 21, for example, a phase shift on the order of 10 .mu.s may exist. As a result, the actual timing of information in line 21 as indicated by the sync separator output differs from the timing indicated by the output of a horizontal PLL. Thus, while the sync separator output accurately indicates the video signal timing for line 21, the horizontal PLL output may not.
The preceding discussion indicates that the sync separator output is the preferred timing reference for purposes of locating the RIC signal in line 21. However, system constraints may dictate that a horizontal sync signal from the horizontal PLL must be used as a timing reference. In this situation, phase shifts between the horizontal PLL output and the video may make location of the RIC signal unreliable possibly causing an inaccurate data slicing level and subsequent corruption of extracted auxiliary data.